Theoretical investigation of Jahn-Teller and pseudo-Jahn-Teller interactions in the ammonia cation

A vibronic-coupling model for the X̃²A₁ and òE states of NH₃⁺ which takes into account all six vibrational degrees of freedom of the four-atomic molecule was developed. The physical significance as well as the deficiencies of the model were assessed by a comparison of the adiabatic potential-energy functions with ab initio data over an extended range of symmetry-adapted valence coordinates. The main result was a semiquantitative description of the band shape and diffuse vibronic structures of the òE photoelectron band of NH₃.